This invention relates to fluid parameter measurement in pipes and more particularly to measuring speed of sound and parameters related thereto of fluids in pipes using acoustic pressures.
It is known that the speed of sound amix of fluids in pipes may be used to determine various parameters of the fluid, such as is described in U.S. Pat. No. 4,080,837, entitled xe2x80x9cSonic Measurement of Flow Rate and Water Content of Oil-Water Streamsxe2x80x9d, to Alexander et al., U.S. Pat. No. 5,115,670, entitled xe2x80x9cMeasurement of Fluid Properties of Two-Phase Fluids Using an Ultrasonic Meterxe2x80x9d, to Shen, and U.S. Pat. No. 4,114,439, entitled xe2x80x9cApparatus for Ultrasonically Measuring Physical Parameters of Flowing Mediaxe2x80x9d, to Fick. Such techniques have a pair of acoustic transmitters/receivers (transceivers) which generate a sound signal and measure the time it takes for the sound signal to travel between the transceivers. This is also known as a xe2x80x9csing-aroundxe2x80x9d or xe2x80x9ctransit timexe2x80x9d method. However, such techniques require precise control of the acoustic source and are costly and/or complex to implement in electronics.
Also, these techniques use ultrasonic acoustic signals as the sound signal measured, which are high frequency, short wavelength signals (i.e., wavelengths that are short compared to the diameter of the pipe). Typical ultrasonic devices operate near 200k Hz, which corresponds to a wavelength of about 0.3 inches in water. In general, to allow for signal propagation through the fluid in an unimpeded and thus interpretable manner, the fluid should be homogeneous down to length scales of several times smaller than the acoustic signal wavelength. Thus, the criteria for homogeneity of the fluid becomes increasingly more strict with shorter wavelength signals. Consequently, inhomogeneities in the fluid, such as bubbles, gas, dirt, sand, slugs, stratification, globules of liquid, and the like, will reflect or scatter the transmitted ultrasonic signal. Such reflection and scattering inhibit the ability of the instrument to determine the propagation velocity. For this reason, the application of ultrasonic flowmeters have been limited primarily to well mixed flows.
Objects of the present invention include provision of a system for measuring the speed of sound of fluids in pipes.
According to the present invention, an apparatus for measuring at least one parameter of a mixture of at least one fluid in a pipe, comprising a spatial array of at least two pressure sensors, disposed at different axial locations along the pipe, and each measuring an acoustic pressure within the pipe at a corresponding axial location, each of said sensors providing an acoustic pressure signal indicative of the acoustic pressure within the pipe at said axial location of a corresponding one of said sensors; and a signal processor, responsive to said pressure signals, which provides a signal indicative of a speed of sound of the mixture in the pipe.
According further to the present invention, the signal processor comprises logic which calculates a speed at which sound propagates along the spatial array.
According further to the present invention, the signal processor comprises logic which calculates a frequency domain representation of (or frequency based signal for) each of the acoustic pressures signals. According still further to the present invention, the signal processor comprises logic which calculates a ratio of two of the frequency signals. In still further accord to the present invention, the sensors comprise at least three sensors.
According still further to the present invention, the pressure sensors are fiber optic Bragg grating-based pressure sensors. Still further accord to the present invention, at least one of the pressure sensors measures an circumferential-averaged pressure at a given axial location of the sensor. Further according to the present invention, at least one of the pressure sensors measures pressure at more than one point around a circumference of the pipe at a given axial location of the sensor.
The present invention provides a significant improvement over the prior art by providing a measurement of the speed of sound amix of a mixture of one or more fluids within a pipe (where a fluid is defined as a liquid or a gas) by using an axial array of acoustic (or ac, dynamic, unsteady, or time varying) pressure measurements along the pipe. An explicit acoustic noise source is not required, as the background acoustic noises within the pipe (or fluid therein) will likely provide sufficient excitation to enable characterization of the speed of sound of the mixture by merely passive acoustic listening.
The invention works with acoustic signals having lower frequencies (and thus longer wavelengths) than those used for ultrasonic meters, such as below about 20k Hz (depending on pipe diameter). As such, the invention is more tolerant to the introduction of gas, sand, slugs, or other inhomogeneities in the flow.
The invention will work with arbitrary sensor spacing and arbitrary flow Mach numbers Mx; however, if the sensors are equally spaced and the axial velocity of the flow is small and therefore negligible compared to the speed of sound in the mixture (i.e., Mach number of the mixture Mx is small compared to one), the speed of sound amix may be determined as an explicit function of the frequency domain representation (frequency based signal) for the acoustic pressure signals at a given evaluation frequency xcfx89.
Since the speed of sound is an intrinsic property of mixtures, the present invention can be used to measure any parameter (or characteristic) of any mixture of one or more fluids in a pipe in which such parameter is related to the speed of sound of the mixture amix, e.g., fluid fraction, temperature, salinity, sand particles, slugs, pipe properties, etc. or any other parameter of the mixture that is related to the speed of sound of the mixture. For example, the present invention may be used to measure fluid volume fractions (or composition or cut or content) of a mixture of any number of fluids in which the speed of sound of the mixture amix is related to (or is substantially determined by), the volume fractions of two constituents of the mixture, e.g., oil/water, oil/gas, water/gas. Also, the present invention can be used to measure the speed of sound of any mixture and can then be used in combination with other known quantities to derive phase content of mixtures with multiple (more than two) constituents.
The present invention allows the speed of sound to be determined in a pipe independent of pipe orientation, i.e., vertical, horizontal, or any orientation therebetween. Also, the invention does not require any disruption to the flow within the pipe (e.g., an orifice or venturi). Further, the invention uses ac (or unsteady or dynamic) pressure measurements as opposed to static (dc) pressure measurements and is therefore less sensitive to static shifts (or errors) in sensing. Furthermore, if harsh environment fiber optic pressure sensors are used to obtain the pressure measurements, such sensors eliminate the need for any electronic components down-hole, thereby improving reliability of the measurement.
Also, a strain gauge (optical, electrical, etc.) that measures hoop strain on the pipe may be used to measure the ac pressure. Fiber optic wrapped sensors may be used as optical strain gauges to provide circumferentially-averaged pressure. Thus, the present invention provides non-intrusive measurements of the speed of sound (and other corresponding parameters), which enables real time monitoring and optimization for oil and gas exploration and production, or for other applications.
The foregoing and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of exemplary embodiments thereof.